DECATUR, Ga.—The Obama administration on Thursday unveiled details of a preschool plan that would mark the biggest expansion of early-childhood education since Head Start was launched nearly 50 years ago.

President Obama visited a pre-kindergarten class at the College Heights Early Childhood Learning Center in Decatur, Ga. on Thursday. Video by WSJ's Peter Nicholas via #WorldStream.

The centerpiece of the agenda is a federal-state partnership that would try to coax states to create preschool slots for all low- and moderate-income 4-year-olds by providing some matching federal funds.

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President Barack Obama, speaking about the plan Thursday after touring a preschool here, said giving every child access to high-quality early education should be a national priority, adding, "Study after study shows that the earlier a child begins learning, the better he or she does down the road."

Mr. Obama's preschool agenda builds on research that shows investment in high-quality early education can students' boost academic achievement and job-earning potential. But it isn't clear how much the proposal would cost, or whether states could build up on a large scale the kind of high-caliber programs that research suggests deliver results.

Administration officials said the price tag would be disclosed in the federal budget next month, but said the move wouldn't add to the nation's deficit.

Still, some Republican lawmakers balked at the potential cost and the idea of a larger federal role in preschool education, while others questioned whether there is enough scientific research to support preschool expansion.

Sen. Johnny Isakson (R., Ga.) said his state's program, which offers free prekindergarten to all 4-year-olds, works because it was "started locally, is funded locally, doesn't have a dime of federal money."

The nation's government-funded preschool programs are a hodgepodge in terms of quality and cost. Mr. Obama said that about one-third of U.S. 4-year-olds are enrolled in high-quality ones, adding that his plan could greatly expand those numbers.

Though the effort has been billed by the administration as "preschool for all," including the middle class, the plan targets disadvantaged children. The program would provide matching funds—although it isn't clear whether it would be dollar-for-dollar—to states that expand preschool slots for families with incomes of twice the federal poverty level or less. States could get extra funds to expand preschool offerings to middle-class families.

States would have to meet certain conditions, such as hiring "well-trained teachers" who are paid comparably to K-12 teachers, adopting rigorous curricula, keeping student-teacher ratios low and assessing youngsters.

The administration would create guidelines for each standard, but states would have some leeway to craft their own.

Nobel Prize laureate James Heckman, an economist at the University of Chicago, praised Mr. Obama's plan and said every $1 invested in quality early-childhood education for low-income children provides a 7% to 10% annual return on investment per child in terms of better education, health and economic outcomes. He said high-quality programs might appear to be expensive, but the costs "are not high in terms of the opportunity costs of not doing the programs."

Maria Fitzpatrick, professor of policy analysis and management at Cornell University, in an emailed news release cautioned against any plan that would expand preschool to all 4-year-olds, saying that "results show that only some children gain—disadvantaged children, particularly those in rural areas—and that the effects fade out over time."

The president's proposal also would increase high-quality early-education programs for infants and toddlers and expand the home-visiting initiative, which sends nurses and other professionals into homes to work with low-income families on parenting and school-readiness skills.

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